The long-term goal of this project is to develop an effective protein replacement therapeutic (PRT) for patients affected with rare forms of Osteogenesis imperfecta, OI-VIII (deficiency in prolyl 3-hydroxylase 1; P3H1) and OI-VII (deficiency in cartilage associated protein CRTAP). These PRTs will integrate the safety and cost advantages of plant-based bioproduction with innovations that enhance delivery and disease correction. OI is a group of genetic connective tissue disorders characterized by low bone mass and increased bone fragility. Current treatment options for OI are limited to bisphosphonates treatments, which inhibit bone resorption but are controversial due to the likelihood of deleterious effects in young patients as well as adults. Developing effective PRTs for these patients is challenging since classic enzyme replacement therapies (ERTs) have been relatively ineffective in correcting bone-related pathologies. PRTs for these rare OI diseases will require efficient delivery to multiple mesenchyme-derived cell lineages as well as effective targeting to rough endoplasmic reticulum (rER) to correct collagen malformation. To expand the delivery of corrective protein to critical cells and tissues, BioStrategies LC has been testing the potential of several plant lectins to function as effective carriers of PRT's into disease cells. These lectins have high affinity for glycoproteins and glycolipids common on mammalian cell surfaces and mediate efficient cellular uptake, transcytosis, and delivery to target tissues. We have demonstrated that one of these lectins, RTB, efficiently carries large fused model proteins into multiple cells of the skeletal systems including chondrocytes, osteoblasts, and mesenchymal cells. We also showed that RTB, modified to contain a KDEL ER-retrieval signal (RTBER), effectively directs payload proteins to the ER following endocytosis. We hypothesize that our fusion proteins will access cells of the skeletal system and will deliver PRTs to correct collagen defects in OI-VIII and OI-VII. The goal of this SBIR Phase I feasibility study is to test the potential of plants to produce RTBER fusions with P3H1 and/or CRTAP that are taken up into osteogenic cells, mobilized to the rER, and function to correct collagen processing defects at the cellular level. Specific aims for Phase I are 1) to produce and characterize RTB:collagen-modifying protein fusions using a transient plant-based expression platform, and 2) to demonstrate product efficacy in vitro by disease phenotype correction in OI cells. Based on successfully meeting the Phase I milestones of in vitro cell correction in either OI-VIII or OI- VI models of rare recessive OI, follow-on Phase II studies would focus on in vivo efficacy in mouse models of these OI diseases. These studies would include short-term biodistribution analyses and skeletal phenotype evaluation following extended replacement therapy treatments in either P3H1 deficient or Crtap-/- mice leading to Phase IIB toxicology assessment and other preclinical endpoints in support of an IND application at FDA.